1. Field of the Invention
This invention relates generally to grinding machines employing a rotary abrasive wheel for carrying out grinding processes and, in particular, the invention relates to a machine and process for automatically controlling and optimizing grinding with a superabrasive grinding wheel by monitoring the normal forces imposed on such grinding wheel.
2. Background Information
It is known in the grinding industry that superabrasive wheels, for example, cubic boron nitride (CBN), will tend to increase in sharpness after trueing as successive workpieces are ground, and thereby will result in higher metal removal rates for an equivalent normal force applied to the wheel, causing variations in sizing and finish.
It is well known that reconditioning (either dressing or trueing) a superabrasive wheel causes it to behave in a dull fashion, since the grit is leveled with the bonding material. If high force is applied to a dull grinding wheel, wheel damage or part burn may occur. Through a variety of grinding actions, the bonding material becomes eroded as the wheel progressively grinds workpieces, exposing new, sharp grit material, allowing room for chip formation, and effectively increasing the sharpness or metal removal ability of the wheel.
One prior art publication which discusses the superabrasive wheel and wheel dressing pattern is U.S. Pat. No. 4,653,235, in which a method is disclosed whereby a watt transducer is utilized to sense grinding wheel drive motor power consumption over a predetermined time interval at a spark-out or dwell portion of the grinding cycle, and the power level during the transition from maximum grinding power to some predetermined lower power level for re-commencing feed is monitored. If the lower power level occurs before the time interval is reached, the wheel is known to have become too sharp for the intended use, and a dressing cycle is initiated to recondition and "dull" the now too-sharp wheel. The broad concept of monitoring wheel drive motor power consumption is well known in machines using conventional grinding wheels, because it is simple and inexpensive and it is possible to determine the tangential force components from the equation: power equals V.sub.s times F.sub.t, where V.sub.s is the grinding wheel speed in meters per second (m/s), and F.sub.t is the tangential force (in Newtons). Power monitors have been very useful as gap elimination devices and crash detectors, and have been used as the bases of adaptive control philosophies. However, power monitors do not lend themselves to measurement of normal force (F.sub.n) which is considered imperative for accurate wheel sharpness measurement. Because the coefficient of grinding can vary considerably between dull and sharp wheels, the tangential force is not a reliable indication of actual normal forces for many working conditions commonly encountered in grinding operations. Moreover, power monitors, such as watt meters and the like, have relatively slow response times.
In evaluating wheel sharpness and controlling the grinding processes, other prior art patents have attempted to teach controlling the wheel feed rate by sensing deflection of the grinding wheel spindle. U.S. Pat. Nos. 3,344,560 (Lillie) and 3,555,741 (Hahn) are two examples of devices which monitor deflection of the grinding wheel spindle to estimate grinding force. The normal force is the principal deflection causing force in the grinding process, particularly with internal grinding machines, and some prior art machines have swiveled the wheelhead to re-align the wheel with the workpiece as the grinding wheel spindle is deflected.
Internal grinding machines are particularly sensitive to deflection of the wheel-supporting spindle, which is generally of small diameter compared, for example, with external grinding machines. Controlled force grinding has been utilized in order to ensure that a known deflection of the spindle could be maintained to control the resulting grinding process. Such controlled force grinding has its problems too, however, as eccentric rotation of the workpiece, or irregularities in the workpiece stock or hardness could cause runout and prevent proper round-up of the workpiece. Heavy damping and/or increased wheel rotation velocities were required to address these problems, which created problems of their own, especially in the context of superabrasive grinding wheels.
An example of additional problems encountered includes post-dressing or post-trueing inefficiencies. As set forth in U.S. Pat. No. 4,628,643 (Gile et al.), following wheel trueing or dressing, the rotational velocity of a superabrasive grinding wheel is reduced to maintain a substantially constant grinding wheel drive power consumption. The desired power level is set by the operator, and the rotational velocity is slowly increased in subsequent grinding operations based upon the average power consumption and feed rate of the previous grind. This process slowly increases the grinding output as the superabrasive wheel becomes sharper (i.e. self-conditions) as described above.
U.S. Pat. No. 4,570,389 (Leitch) addresses post-dressing or post-trueing use of non-superabrasive wheels. The inefficiencies of this device are similar to the Gile device, as following dressing, Leitch teaches that the first grind is to be undertaken at a reduced, pre-set feed rate because non-superabrasive wheels are sharpened by the dressing process. Perceived sharpness of the wheel is calculated by monitoring changes in horsepower or normal force between two successive grinds, and over time the feed rate is increased to the point where the wheel can maintain its sharpness. The increased feed rate tends to self-sharpen the non-superabrasive wheel enough to offset the dulling effects of attrition at the optimum feed rate. However, this slow build up of the feed rate is inefficient. Moreover, this technology is not easily adapted to superabrasive grinding where in order to maintain substantially constant grinding force, the feed rate must be reduced to compensate for the self-sharpening characteristics of superabrasive wheels.